The maximum mean body weights of rats attained during the course of two-year studies increased by about 20% from 1975 to 1985. Higher body weights will lead to increases in the incidences of mammary tumors, pituitary tumors, and possibly other tumors. Modification of diet may slow the growth and lower the maximum body weight attained which in turn may decrease the incidences of spontaneous tumors. A lower protein diet may decrease the incidence and severity of kidney disease. The purpose of this study is to determine the feasibility of 13% protein and 20% fiber diet (NTP-90) in lowering the maximum body weight and decreasing the severity of nephrosis in rats and mice in comparison with high protein, low fiber NIH-07 diet. Groups of 60 male and 60 female F344 rats and B6C3F1 mice received NIH-07 or NTP-90 diets for two years. Body weights, feed consumption and clinical chemistry parameters were determined at selected intervals. Tissues were collected and evaluated for nonneoplastic and neoplastic changes at the end of the study.